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Micro-scaled progressive sheet metal forming is a promising process for producing bulk microparts, given its advantages of high efficiency and low cost. To enhance forming quality and efficiency, it is important to have an in-depth understanding of the forming mechanism and model the forming process and fracture behavior accurately. However, the prediction of fracture formation in sheet materials at the micro-scale has not yet been well explored, and thus current knowledge is not sufficient to support the continued development and application of microforming technology. This study investigated progressive sheet forming of magnesium-lithium alloy sheets of different grain sizes to produce bulk microparts directly from sheet metal via shearing, extruding, piercing, and blanking. Using the Gurson–Tvergaard–Needleman (GTN) damage model, the effects of the size factor on the formation and evolution of voids were considered, and the shear-modified GTN model was established by combining Thomason's and Lemaitre's damage mechanics models. The modified model could predict not only the ductile fracture behavior dominated by tension under high-stress triaxiality at the micro-scale, but also the damage behavior controlled by shear deformation under low-stress triaxiality. The progressive forming process was simulated using the modified model, which was verified by experimentation and simulation. Comparisons of the experiments and simulations revealed the size effects on the forming defects and fracture behaviors of microparts during progressive sheet forming. The results show that the stress during deformation is mainly concentrated at the edge of microparts, and irregular geometric defects including burr, rollover, incline, and bulge become deteriorated with the increase of the initial grain size. This study enhances the understanding and prediction of ductile fracture in the micro-scaled progressive forming of sheet metals. [ABSTRACT FROM AUTHOR]

Although researchers have extensively investigated the dynamic mechanical properties of metallic materials at the macro-scale in terms of deformation mechanisms, strain rate strengthening and fracture mechanisms, they have rarely investigated the dynamic mechanical properties affected by size effects at the micro-/meso-scales. In this study, experiments on quasi-static compression and the split-Hopkinson pressure bar (SHPB) were conducted using oxygen-free high-conductivity (OFHC) copper with different geometrical and grain sizes to explore the size effects on various dynamic mechanical properties at the micro-/meso-scales. The experimental results showed that the quasi-static and dynamic mechanical properties of OFHC copper are affected by size effects at the micro-/meso-scales. In particular, OFHC copper was found to exhibit strain rate strengthening effects at the micro-/meso-scales, and the presence of micro-cracks was observed in the SHPB experimental specimens. In this study, after proposing a modified Johnson-Cook (J-C) constitutive model based on the surface layer model, we analysed the average relative error of the modified model and the original constitutive model. The influence of the size effect on the mechanical properties of materials is taken into account in the new modified constitutive model. The results of relative error analysis showed that the average relative errors for the original J-C model ranged from 3.2% to 18%, while the modified J-C model's results ranged from 2.2% to 11%. The accuracy of the modified model is greatly improved. Following finite element analysis based on the modified J-C model and the original model, the results showed that the modified J-C model agreed well with the experimental results. [ABSTRACT FROM AUTHOR]